It is known that in many interventions carried out by dental surgeons on a person, notably when giving a local anaesthesia prior to the extraction of a tooth, or more generally for the administration inside the dental mucosa of medicinal substances adapted to a determined treatment, it is necessary to inject a liquid product in a quantity which is controlled with precision, with a frequency which, eventually, can be relatively high during the intervention. To this effect, apparatus adapted to dental practice and presenting an adequate ergonomy for facilitating their handling by the practitioner are known, but their shape is nevertheless derived more or less directly from standard syringes where the liquid, pushed by a piston moving in a cavity, is driven out through the inner channel of a hollow needle, which was previously introduced via its open and cutting end into the mucosa, at the place where the product has to be injected. However, these standard apparatus are not very practical and require notably a consumption of needles the replacement of which is essential after each intervention, and even after each injection. Moreover, the use of a needle fixed at the end of a syringe always has a traumatic effect on the patient; it is borne by some of these patients only with great difficulties. On the other hand, the use of an apparatus of this kind requires from the practitioner great skill, above all if the site of the gum where the product has to be injected is not easily accessible; for example, inside and on the bottom of the buccal cavity or in the vicinity of the ligual face of the incisive teeth, the space requirement of the needle makes the operation all the more delicate because the chosen region is so difficult to reach.
Finally, there are the risks resulting from a possible contamination of the patient, and even of the practitioner, through the blood which often accompanies the penetration by the needle of the mucosa, this being another, still more important, disadvantage of using standard injection apparatus employing such needles.
In order to remedy this disadvantage, it has already been proposed to replace the hereabovementioned syringe with a needleless injector device, of the general type of those used in the care technique known as mesotherapy. However, with the corresponding apparatus, the cocking of the piston which creates the extremely fine liquid jet directly penetrating the gum due only to the force of its impact in contact with said gum is carried out by means of a manual lever which has to be operated at each injection. Hence, it is a heavy apparatus, difficult to operate, and, above all, does not allow varying the dose to be injected, which is always the same, since the piston stroke is fixed. Moreover, for the injection of larger doses, in order, for example, to carry out a deep local anaesthesia, the penetration of the useful quantity of product requires carrying out a series of successive operations of the apparatus cocking lever, which makes the apparatus neither practical nor efficient.
Finally, another disadvantage results from the fact that in some embodiments, the liquid to be injected, prior to the use of the apparatus, has to be transferred in a reservoir mounted inside the apparatus, hence causing a problem for cleaning and eventually sterilizing the latter when there is a change of product, with, moreover, the disadvantage of the waste of the product, the excess of said product normally having to be discarded each time.